[colug-432] best ID function

[R P Herrold]

On Mon, 13 May 2013, Rick Troth wrote:

Some further definition of the 'goodness function' are needed 
to understand what 'best' means here ... Here is an entrant 
for 'best' as to the second example, It is one I discussed in 
a call last week, which PMman supports well.  I need to write 
a blog post ...

> In this case, I need the username, not the UID.

getent if one has an ability to query up into the parent, but 
in a chroot ('I'm writing a jail ...') environment, I am not 
clear that this works portably

I suspect your are writing code with the assumption of a 
network environment, for a presentation in June you may be 
doing ...

As such assuming network access, (and having the nice property 
of having 'zero forward knowledge' being exposed beyond the 
value of a 'general case' payload, here: userid [1]), a salt, 
a shared secret, and a unique per application set of keying, 
of a symmetrically encoded value placed into DNS through a DNS 
management API, It is pretty easy to write (basically being an 
implemention grafting a DHT facility [2] on top of DNS)

Meta-code follows.  Plaintext DNS transfers are fine across 
untrusted networks, as the strength is needed only at the 
(assumedly protected) endpoints

Only a few minimal transactions need be supported outside of 
the fairly trivial crypto transforms (all transforms shown in 
the worked example below):
 	write
 	read
 	clear

The calling side either re-uses a salt from an agreed 
location, or a pre-known one, or if it knows that it is a one 
time use, could place one at a temporary UUID name 'A' with 
a 'write' transaction

The called and calling side each know a shared secret (similar 
to Radius' approach), and here, a 'passphrase' as well

The calling side places an encrypted value of the plaintext to 
be transferred (here, the username) at either an agreed 
location, or if it knows that it is a one time use, could 
place it at a temporary UUID name 'B' with a 'write' 
transaction

The calling side hands the the value of B (and optionally A) 
to the called side.  If the transaction needs to cross an 
unprotected network, gpg clearsigning the transaction comes to 
mind.  Not relevant in a chrooted environment

The called side uses B (and optionally A), and the shared 
secret to product the plaintext back with a 'read' 
transaction

Optionally the called side, or the passage of time, or the 
calling party when it gets a return code, or whatever can 
'wipe' the transient data with 'clear' transactions toward B 
and A

Logging happens on the DNS server, and as otherwise desired. 
'False flag' MitM pollution is detectable there.  The DNS 
server lacks other sufficient information beyond the effective 
IPs of its querying counterparties, such that it has no way to 
decode (decrypt) the transferred payload data; as it lacks 
both the shared secret, and the passphrase

This implementation does not contain a MAC and is subject to 
replay attacks -- thus the wipe -- but that is out of scope 
here and as noted trivially solved.  A and B need not be 
published in the same DNS domains, of course, to taste as to 
privacy

--
end
==================================
  .-- -... ---.. ... -.- -.--
Copyright (C) 2013 R P Herrold
       herrold at owlriver.com
    My words are not deathless prose,
       but they are mine.


[1] http://en.wikipedia.org/wiki/Zero-knowledge_proof
[2] http://en.wikipedia.org/wiki/Distributed_hash_table


Worked example:

1. Retrieve the salt from:
 	dig -t txt salt.owlriver.net

[herrold at centos-6 ~]$ dig -t txt salt.owlriver.net | grep TXT
;salt.owlriver.net.             IN      TXT
salt.owlriver.net.      296     IN      TXT     "0a07"

and which was produced:

[herrold at centos-6 ~]$ openssl rand -hex 2
 	0a07


2. The client specific shared secret is:
 	deadbeef
and the shared passphrase is:
 	asdf


3. The plain text was at:
 	dig -t txt colug1.owlriver.net

4. The cipher text is at:
 	dig -t txt colug2.owlriver.net

[herrold at centos-6 ~]$    dig -t txt colug1.owlriver.net | grep TXT
;colug1.owlriver.net.           IN      TXT
colug1.owlriver.net.    295     IN      TXT     "Hello Colug"
[herrold at centos-6 ~]$    dig -t txt colug2.owlriver.net | grep TXT
;colug2.owlriver.net.           IN      TXT
colug2.owlriver.net.    300     IN      TXT
 	"U2FsdGVkX18KB96tvu8AAF8SnDkSeyjhd4stUIrWSmA="
[herrold at centos-6 ~]$


5. The transforms (here plain old DES ... dial up to taste) 
are:

[herrold at centos-6 ~]$ echo "Hello Colug" | openssl enc -e -des
 	-S '0a07deadbeef' -a -pass pass:asdf
U2FsdGVkX18KB96tvu8AAF8SnDkSeyjhd4stUIrWSmA=
[herrold at centos-6 ~]$ echo
 	"U2FsdGVkX18KB96tvu8AAF8SnDkSeyjhd4stUIrWSmA=" | openssl
 	enc -d -des -S '0a07deadbeef' -a -pass pass:asdf
Hello Colug